Exporting Node Textures / Texture Baking
PBR and texture baking are simple when you organize your concepcts correctly.
First thing you have to UV Unrwrap your model because you're telling how blender and Unity will shrink-wrap the textures to your model.

When you are working with a PBR Workflow you'll have in most times 4 maps : 


1 ALBEDO/DIFFUSE
Is a color map, represents the colors,logos,textures etc... in your object;
2 METALNESS (IN SOME CASES SPECULAR MAP IS ALSO USED)
How metal some object is,and where it's metal or where isn't;
3 NORMALS
Normal maps is a technique used for faking the lighting of bumps. Are commonly used to represent a HighPoly mesh into a low poly, or add details visible beacuse it says to render where the light has bumps or not.
4 ROUGHNESS
How rough a object is,how "reflective" it's...


Here's a example of maps :



Then rendered :



But you also could've been using AO,Emissive,Opacity and heigmaps.

They act like informations to the render know how to properly render your object correctly in any light circunstances. Your light in baking process is irrelevant.

But the light when you RENDER, will matter. The render uses your PBR maps to know how light will affect the object so you can get better results based on real life's physics.

Baking is the process where you make the maps itself.

Here's a example to texture Baking a NORMAL MAP :

There are 2 objects a low poly(not that low) and a High poly



When you bake the normal map,you'll got a map to use in low poly mesh to fake light bumps into it and get a smoother result.

Hope it will help you in a basic level, if something isn't clear, please let me know friend. 

In addition ,Blender 2.79 will have a native solution for PBR Workflow in cycles, check it Later.

Principled Node - Blender Manual